1. Field of the Invention
This invention relates to holography and more particularly, to preventing intermodulation distortion of an image reconstructed from a phase hologram.
2. Description of the Prior Art p A phase hologram is a diffraction grating recorded by modulating either the index of refraction or the thickness of a recording medium with a desired interference pattern. The desired interference pattern is formed at the medium by an interaction of two mutually coherent beams of recording light. One of the beams, referred to as an object beam, has rays that are spatially modulated in either phase of amplitude to provide a representation of an image; the other beam is referred to as a reference beam. A reconstruction of the image emanates from the hologram in response to a transmission thereto of a reconstructing beam.
In addition to the desired interference pattern, an undesired interference pattern if formed by rays of the object beam that interact with each other at the medium. The undesired interference pattern is a cause of what is known as intermodulation distortion during a reconstruction of the image. The intermodulation distortion is a dominant form of distortion when the thickness of the recording medium is less than a wavelength of the recording light.
The intermodulation distortion may be reduced by maintaining the diffraction efficiency of the hologram at a low level, typically in a range of one to three percent. The low diffraction efficiency causes the reconstruction of the image to be of low intensity compared with the intensity of the reconstructing beam. Therefore, the low diffraction efficiency is undesirable.